1. Field of the Invention
The present invention relates to a sapphire single crystal provided with a smooth cleavage plane, more particularly, to a single crystal sapphire substrate easier to cleave, so as to be used as a substrate, of thin film growth, such as semiconductor or the like, for electronic parts or structure parts, and to a method of working the same. Furthermore, the invention relates to a semiconductor laser diode using such a single crystal sapphire substrate and a method of manufacturing the same.
2. Prior Art
The sapphire single crystal, i.e., Al2O3, is used for usage of wider range, because it has higher clear-degree, hardness and comparatively smoother plane. The sapphire of the single crystal is pulled, growing a seed crystal in contact with the surface of the molten alumina to produce the single crystal into a larger monocrystal, so as to generally work the single crystal into the desired shape.
A method of working the sapphire single crystal comprises mechanical working such as grinding, abrading or the like with the use of diamond grindstone, diamond grinding grains, chemical etching working with the use of corroding phenomena, and furthermore, enlarging a fine microcrack, formed on the surface with a diamond needle point pen tip, so as to effect a cleaving, breaking operation.
With the working methods as described above it is difficult to obtain a smooth surface in a short time. The mechanical working with diamond grindstone is longer in working time and is higher in working cost. The chemical etching operation, which has an advantage of easily compensating the sapphire face smoothly, takes approximately 10 hours for effecting etching operation 1 mm in thickness, and it is hard to obtain a smooth face of sub-micron level. Also, the breaking, working method with the diamond needle point pen tip is worse in the accuracy of the worked surface, and it is difficult to obtain a smooth face.
The sapphire single crystal is used to form a laser light emitting element, composed of multiple semiconductor layers, on the surface of the substrate. In the semiconductor laser diode, whose schematic structure is shown in FIGS. 6 and 7, a semiconductor multilayer 3 forming the laser light emitting element is disposed on a buffer layer 2 on the surface of the single crystal sapphire substrate 1 to act as a resonator for the laser beam between a pair of opposite end faces 3a of the multilayer 3. The pair of end faces 3a are smooth reflection end faces extremely high in precision and simultaneously, the parallelism of both the faces is required to be extremely high, so as to improve the oscillation efficiency.
Although a method of growing a semiconductor thin film on the major plane of the single crystal sapphire substrate to form patterns, and then, dividing the substrate into a chip shape is used to produce, generally, the semiconductor element including a laser diode, the above described conventional methods are hard to use with precision, so as to reduce the yield of the chips. Although the chip division plane is used as the reflection end face, especially when the semiconductor thin film is grown further into a multilayer on the substrate, and then, divided into chips with each substrate as the semiconductor laser diode, the higher precision of a sub-micron level required for the reflection end face cannot be achieved by the above described working method.
As for this problem, although Japanese Patent Application Laid-Open A7-27495 discloses that the single crystal sapphire substrate is cleaved, divided into parallel to the axis C  less than 0001 greater than  of the sapphire crystal, this method is incapable of obtaining a smoother surface of high precision.
An object of the invention is to provide, first, a sapphire single crystal having a smoother division plane higher in precision, and a single crystal sapphire substrate.
Another object of the invention is to provide a dividing method for a sapphire single crystal capable of forming such a precise division plane in short time on the sapphire single crystal.
Still another object of the invention is to provide a semiconductor laser diode provided a precise smooth plane on the end face of the resonator of the single crystal sapphire substrate.
A further object of the invention is to provide a method of forming a precisely smoother plane on the resonator end face of the single crystal sapphire substrate to manufacture a semiconductor laser diode.
The invention applies a plane R to the formation of the smoother surface of the sapphire single crystal with the use of a fact where the plane R of the sapphire single crystal is easier to cleave and the cleavage plane is a smooth plane of higher precision. Now, the R plane is referred to as the (1102) plane indicated with hexagonal indices.
The sapphire single crystal having such a plane R for the cleavage plane includes a sapphire tool, the other structure members or a single crystal sapphire substrate having the cleavage plane on its side face. Such a sapphire single crystal plate is used as a single crystal sapphire substrate whose major plane has elements such as semiconductor elements, functional elements and so on.
The invention detects the plane R existing within the sapphire single crystal body to cleave along the plane R, so as to divide the single crystal body and have it on the surface of the smooth plane R on the single crystal body. To simplify division of the sapphire single crystal body or the substrate by the cleavage, the invention includes an index, for controlling the plane R in the dividing, with the reference plane related to the plane R being formed in, for example, the single crystal body or, for example, the edge portion of the substrate, by previous specification of the plane R of the single crystal sapphire in the X-ray crystal study.
More concretely, in the example of the single crystal sapphire substrate, the reference plane which is substantially parallel to or substantially vertical to the plane R is provided on the periphery of the substrate. For the cleaving operation, a method is adopted for forming a linear crack which is parallel to or vertical to the reference plane of the substrate is formed, and having the microcrack line as a start point to develop a crack in the thickness direction for a cleaving operation.
Also, in the single crystal sapphire substrate of the invention, the single crystal sapphire substrate where the semiconductor multilayer is formed is cleaved, separated along the plane R to form a cleavage plane connected with the semiconductor multilayer and the substrate in a substrate where a laser diode is formed for forming the semiconductor multilayer on the major plan of the substrate. The cleavage plane of the semiconductor multilayer is also an extremely smooth plane. The invention uses it for the reflection plane for laser resonator use of the semiconductor multilayer.
A semiconductor laser diode using the single crystal sapphire substrate of the invention comprises a semiconductor multilayer composing a laser element on the major plane of the single crystal sapphire substrate, and is characterized in that two reflection end planes for composing the resonator of the laser beam in the multilayer are cleaved planes connected with the cleaved plane along the plane R of the crystal of the single crystal substrate.
A method of manufacturing a laser diode of the invention comprises steps of using a single crystal sapphire substrate provided with the reference plane, forming the semiconductor multilayer for emitting the laser beam the multiple on the major plane, then cleaving the single crystal sapphire substrate and the semiconductor layer along the R face with the reference plane of the sapphire single crystal as an index. By this method, they are divided quickly and easily into many laser diode chips provided with smooth cleaved face on both the end planes of the semiconductor multilayer.